Startling new research into mobile phones claims they may reduce a man's sperm count by up to 30%.

The study is the first to indicate that male fertility may be damaged by the radiation give off by the handsets.

Men who carry a mobile phone in a belt holster or their trouser pocket have been warned they are the ones most at risk from the threat.

Even many of those sperm that did survive exposure to mobiles were partially damaged, further reducing fertility.

But while the research suggests an effect, the scientists say further work will need to be done to confirm the finding and establish how it might happen.

The paper by Dr Imre Fejes of the obstetrics and gynaecology department at the University of Szeged in Hungary will be released at a conference in Berlin, Germany, on Tuesday.

Dr Fejes wrote: "The prolonged use of cell phones may have a negative effect on spermatogenesis (sperm production) and male fertility, that deteriorates both concentration and motility."

Unlike previous studies, says The Sunday Times, researchers believe phones may cause damage while in stand-by mode as they still make regular transmissions to the nearest radio masts.

It had been assumed such transmissions were too short to cause harm.